In a typical small-scale drill press, a drill bit is held in a chuck or collet. The bit is then rotated and brought to bear against a workpiece. A typical lathe is similar, but it rotates the workpiece instead of the machine-tool. In these machines, keeping the machine-tool aligned with the workpiece is important but fairly straightforward.
The situation changes with the more complex machines used in industry. Often, these machines include a rotating turret disk that holds a number of toolholders. Each toolholder is available to hold one machine-tool. In operation, the turret disk is rotated so that a toolholder holding a machine-tool appropriate for the next machining step is moved into place. The machine-tool is then brought to bear on a workpiece held by a rotating spindle. These multiple-toolholder machines are very useful because they allow an operator, whether a human or a computer, to rapidly switch from one machine-tool to another without having to remove one machine-tool and then insert another. However, the movement of the toolholders into position and then out again has implications for alignment.
Ideally, once a machine-tool held in a toolholder is properly aligned with the axis of rotation of the spindle that holds the workpiece, this machine-tool can be moved away and back again without affecting the alignment. Reality intervenes, however, and the pressures and vibrations of the working machine, and wear on the machine-tool itself, gradually cause the machine-tool to work out of alignment. This is a well known problem, and for it there exist well known fixes. The primary fix involves expert aligners who come out and fix the alignment of the toolholders. As a job requiring great expertise, this is an expensive process. During the alignment, the multiple-toolholder machine may be out of operation for a significant amount of time. After all that, the machine-tools eventually go out of alignment again, and the expensive and time-consuming process must be repeated.